It’s hard to believe, but winter is just around the corner, and with it comes a new wave of HVAC claims. A common failure that aging HVAC systems suffer in fall and winter months is the breakdown of the heat exchanger due to
thermal fatigue. To some degree, all metals that are repeatedly
heated and cooled are subject to thermal fatigue, a cyclic stress
that weakens the metal to an eventual point of failure. The degree
to which metals are affected by thermal fatigue includes such
stressors as: How often the metal heats and cools, how quickly
the temperature of the metal changes, and the composition of
the metal. When metal fails due to thermal fatigue, it most often
occurs when the metal is hot, and presents itself in the form of a
crack, which grows and deepens over time.

One HVAC component particularly susceptible to thermalfatigue is the system’s heat exchanger. These are found in everygas- or oil-fired split system or packaged unit application, andthey provide an efficient method of transferring the heat createdby the burners to the supply air, whilesegregating the combustion gassesfrom the air itself. As seen in Figure 1,the typical heat exchanger comprisesseveral metal tubes wrapped in a ser-pentine fashion that are connectedon one end with the system’s burnersand combustion chamber, and on theother end to a vent or exhaust flue. Asthe supply air is pushed by the bloweracross the heat exchanger tubes, theheat transfers away from the heat ex-changer to the passing air, followingthe second law of thermodynamics.In HVAC heat exchangers, thermalfatigue is accelerated by the corrosivecombustion gasses that pass throughthe heat exchanger tubes on their way to the exhaust flue andby condensation that may collect in the heat exchanger duringheat transfer. Heat exchanger cracks can range in size from small,almost imperceptible fissures, to gaps wider than a half inch ormore. And when heat exchangers do crack, the failure not onlyresults in increased claim settlement amounts (due to the costlyimpact of the failure), but may also introduce toxic gasses intothe insured’s supply air.

Such was the case with one commercial rooftop packaged unit
installed in 1986, which suffered heat exchanger failure due to
a lack of routine system maintenance and age-related wear and
tear (see Figures 2 and
3). In this case, the elbows of the heat exchanger tubes had been subjected to a combination of moisture,
corrosion and repeated heating and cooling, and thus began to
rust and deteriorate over time. The tubes ultimately failed at this
elbow location (a common failure position found during field inspections), and the air being supplied to the commercial building
was contaminated by the exhaust fumes.

Unfortunately, the heat exchanger crack was not immediately discovered by the insured, exposing unwitting tenants with
a steady supply of toxic air. Only after repeated flame rollouts
(which were attributed to the pressure changes caused by the
holes in the heat exchanger tubes) melted the system’s control
wiring — which led to total system failure — was the true scope
and impact of the failure recognized. The insured was fortunate,
in fact, that the flame rollout did eventually cause the system to
fail. Had the system controls not been destroyed, the supply air
would have continued to be polluted with toxic fumes. These
toxic fumes could have included carbon monoxide, which can
cause illness or even be fatal.